Carbon nanotubes litz wire for low loss inductors and resonators

ABSTRACT

An upper frequency-range circuit ( 160 ) includes a load element ( 168 ) exhibiting a capacitive load impedance. A first matching network ( 166 ) includes at least one nano-scale Litz wire ( 100 ) inductor. The first matching network ( 166 ) exhibits an inductive reactance that nominally matches the capacitive load reactance. An electrical conductor for providing connections for radio-frequency signals includes a plurality of nano-scale conductors ( 120 ) that are arranged in the form of a Litz wire ( 100 ). In one method of making a Litz wire ( 142 ), a plurality of carbon nanotubes ( 144 ) is placed on a substrate ( 146 ). The carbon nanotubes ( 144 ) are woven according to a predefined scheme so as to form a Litz wire ( 142 ). An inductor may be formed by manipulating the Litz wire ( 100 ) to form a coil ( 150 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electronic wiring components and, morespecifically, to a nano-scale conductor Litz wire used in ultra-highfrequency applications.

2. Background of the Invention

Alternating electric currents tend to distribute themselves within aconductor so that the current density near the surface of the conductoris greater than the current density nearer to the center. Thisphenomenon is often referred to as the “skin effect.” In high frequencyapplications, the skin effect becomes more pronounced, especially insuch devices as inductors and transformers, resulting in a substantialpower loss.

The skin effect may be reduced by use of a Litz wire (short for“Litzendraht wire”), which consists of a plurality of relatively thinwires that are individually coated with an insulating film and thenwoven or braided. By doing so, a Litz wire increases the overall surfacearea of the conductor, thereby reducing the overall skin effect in theresulting cable. Also, in a Litz wire, the ratio of impedance toresistance is increased, relative to a solid conductor having the samecross sectional area, resulting in a higher Q factor at higherfrequencies. Q factor is a measure of energy dissipation in resonantsystems in which a higher Q factor indicates less energy dissipation.

At high UHF frequencies, conductor losses increase due to the skineffect, even with conventional copper Litz wires. This is because at UHFfrequencies, the skin effect in the individual strands become sopronounced that the overall power loss becomes significant. One solutionto this problem would be to create list wires using ultra-fine copperwires. However, the required diameter of a copper wire used as acomponent wire of a Litz wire for UHF applications would be so fine thatit would not be able to withstand the stresses imparted on it as aresult of conventional manufacturing processes and ordinary use.

As antennas used in communications are miniaturized for inclusion inintegrated circuits, they exhibit proportionally more capacitance thantheir larger-scale counterparts. This capacitance must be matched withan inductance for such antennas to be useful in communications circuits.Currently, the minimum size of an antenna is limited by the amount ofinductance that can be used to match the capacitance exhibited by theantenna. A similar problem exists when trying to miniaturize amplifiers.However, no inductor exists that is both small enough to workeffectively with a miniature amplifier or antenna and capable ofproviding sufficient inductance to match the capacitance of theminiature antenna or amplifier.

Carbon nanotubes are molecular-scale tubes of graphitic carbon. Theywere first synthesized using a carbon arc evaporator in 1991. Initially,such tubes contained at least two layers and often many more.

A class of carbon nanotube, the single-walled nanotube (also referred toas “SWNT”), was discovered in 1993. Single walled nanotubes aregenerally narrower than the multi-walled tubes and have a range ofexceptional physical properties, both in terms of strength and in themanner in which they conduct electricity.

Carbon nanotubes employ a graphitic molecular structure and employ sp²carbon bonds, in which each atom is joined to three neighbors. A carbonnano-tube is essentially a rolled-up single graphitic layer in which theatoms along one edge of a graphitic layer bond to their correspondingatoms along the opposite edge.

There are several configurations of carbon nanotubes, that depend on theamount of heliecity (degree to which a helical structure is exhibited)found in the structure of the atoms in the nanotube. One configuration,referred to as the “armchair” configuration, exhibits little heliecity;whereas another configuration, referred to as the “chiral” configurationhave six atom carbon hexagons arranged in a helix. The structure of thenanotube determines some of its physical properties.

Currently, the arc-evaporation method produces the highest qualitynanotubes. It involves passing a large current through two graphiteelectrodes in an atmosphere of helium. Some of the graphite in theelectrodes vaporizes and then condenses on the walls of the reactionvessel and on one of the electrodes (the cathode). Single-wallednanotubes are produced when cobalt and nickel is added to one of theelectrodes (the anode).

Carbon nanotubes may also be made by passing a carbon-containing gas ata suitable temperature and pressure over nano-scale particles of acatalyst (such as iron, nickel or cobalt). The particles help thebreakdown the gaseous molecules into carbon, thereby causing a tube togrow with a metal particle at the tip. Another method for making carbonnanotubes involves using a powerful laser to vaporize a metal-graphitetarget. The catalysis method allows for accurate placement of thenanotubes and more direct control over the manner of their growth.

The strength of the carbon-carbon bonds in a nanotube gives them amazingmechanical properties. For example nanotubes can have a stiffness thatis five times more than that of steel and have a tensile strength thatis about fifty times that of steel. Yet on a per unit volume basis, theyweigh about one-fourth that of steel.

Electrically, a carbon nanotube can act as either a conductor or asemi-conductor, depending on its configuration. In certainconfigurations, they move electrons through a ballistic charge transportmechanism. As a result, they experience minimal electrical resistanceand, thus, generate a minimum amount of heat when conducting.

Therefore, there is a need for a conductor that exhibits minimal skineffect while being able to withstand ordinary stress from manufacturingand use.

There is also a need for a Litz wire that may be employed in passivecomponents used in integrated circuits.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the present inventionwhich, in one aspect, is an upper frequency-range circuit that includesa load element exhibiting a capacitive load impedance. A first matchingnetwork includes at least one nano-scale Litz wire inductor. The firstmatching network exhibits an inductive impedance that nominally matchesthe capacitive load impedance.

In another aspect, the invention is an electrical conductor forproviding connections for radio-frequency signals. The conductorincludes a plurality of nano-scale conductors that are arranged in theform of a Litz wire.

In another aspect, the invention is an inductor that includes a Litzwire. The Litz wire consists of a plurality of carbon nanotubes. TheLitz wire is formed as a coil.

In another aspect, the invention is a method of making an inductor inwhich a plurality of carbon nanotubes is placed on a substrate. Thecarbon nanotubes are woven according to a predefined scheme so as toform a Litz wire. The Litz wire is formed into a coil.

In yet another aspect, the invention is a method of making a conductorfor transmitting radio-frequency signals. A plurality of carbonnanotubes is placed on a substrate. The carbon nanotubes are wovenaccording to a predefined scheme so as to form a Litz wire.

These and other aspects of the invention will become apparent from thefollowing description of the preferred embodiments taken in conjunctionwith the following drawings. As would be obvious to one skilled in theart, many variations and modifications of the invention may be effectedwithout departing from the spirit and scope of the novel concepts of thedisclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIG. 1 is a side elevational view of a Litz wire constructed from carbonnanotubes.

FIG. 2 is a cross sectional view of the Litz wire shown in FIG. 1, takenalong line 2-2′.

FIG. 3 is a cross sectional view of a many-stranded Litz wire.

FIG. 4 is a cross-sectional view of a Litz wire having a rectangularcross section.

FIG. 5 is a schematic diagram of a cantilever-based probe arrangingcarbon nanotubes.

FIG. 6 is a plan view of a planar spiral inductor made from a Litz wire.

FIG. 7 is a schematic diagram of an active circuit that includesnanotube Litz wire matching networks.

FIG. 8 is a schematic diagram of a passive circuit that includes ananotube Litz wire matching network.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. As used in the description herein and throughout the claims,the following terms take the meanings explicitly associated herein,unless the context clearly dictates otherwise: the meaning of “a,” “an,”and “the” includes plural reference, the meaning of “in” includes “in”and “on.”

Also, as used herein “upper frequency range” includes frequencies in theUHF and SHF ranges. Generally, a frequency above 300 MHz (such as afrequency in the range of from 800 MHz to 10 GHz) would be included inan upper frequency range.

As shown in FIGS. 1 and 2, one embodiment includes a Litz wire 100 thatis woven from a plurality of nano-scale conductors 120. The nano-scaleconductors 120 could be nanotubes (such as carbon nanotubes) or could beother types of nano-scale conductors, including nano-ribbons, nano-rodsand nano-wires, or a combination thereof. The materials used to make thenano-scale conductors can include carbon graphene structures, metals,metal oxides and crystalline materials. Typically, in a carbon nanotubeembodiment, armchair-type single-walled nanotubes (SWNTs) would be usedin the Litz wire 100 if ballistic charge transport in the nanotube isdesired. However, other types of nanotubes (e.g., chiral-type andmulti-walled) could be used to take advantage of their respectiveelectrical properties in relation to certain embodiments.

A helically woven “rope-twist” arrangement of a Litz wire 100 is shownin FIGS. 1 and 2. Several nano-scale conductors 120 are first woven ortwisted into primary strands 110. The primary strands 110 are thentwisted or woven together to form the Litz wire 100. Many differenttwisting schemes and weaving patterns may be employed so long as each ofthe individual nano-scale conductors 120 has essentially the same amountof surface area exposed in the outermost surface of the Litz wire 100.

A more complex Litz wire 130 is shown (in cross-section) in FIG. 3. Inthis embodiment, six strands 132, each formed from six nano-scaleconductors 134, are twisted together to form the Litz wire 130. Arectangular weave embodiment of a Litz wire 136 is shown (incross-section) in FIG. 4. In this embodiment, the individual nano-scaleconductors 138 may be woven together, such as with a “basket weave”pattern, and then pressed to form a rectangular cross-section.

One method of making a Litz wire 142 is shown in FIG. 5, in which aplurality of nano-scale conductors 144 is disposed on a substrate 146.Each of the nano-scale conductors 144 is manipulated by acantilever-based probe 140 to weave or twist them into a Litz wire 142.The manipulation of the nano-scale conductors 144 is done usingconventional atomic force microscopy techniques. If a massively-parallelarray of cantilever-base probes 140 is used, many Litz wires can be madesimultaneously. Once the Litz wires are formed, they may be arrangedinto a useful structure, such as a planar spiral coil 150 (which may beused as an inductor), as shown in FIG. 6.

One embodiment of an active upper frequency range circuit 160 is shownin FIG. 7. In this circuit 160, a signal is fed into a miniatureamplifier 164 which feeds a load 168 having an impedance. Typically, theamplifier 164 would be constructed of nano-scale semiconductors, such astransistors, and would have a relatively high capacitance associatedwith the semiconductors. The impedance of the signal is matched to inputimpedance of the miniature amplifier 164 with an input nanotube Litzwire matching network 162. Similarly, the output impedance of theminiature amplifier 164 is matched to the impedance of the load 168 withan output nanotube Litz wire matching network 166. The matching networks162 and 166 include nanotube Litz wire inductors, which may be in serieswith or parallel to the signal (or a combination of both, depending uponthe specific application).

An embodiment of a passive circuit 170 is shown in FIG. 8. In theexample shown, a signal is fed from a source to a miniature antennaelement 174, such as a miniature dipole. Because of its scale, theantenna element 174 would exhibit relatively high capacitive reactance.A corresponding inductive reactance is required and, therefore , aninductive network using a nanotube Litz wire matching network 172 isused to cancel the reactance and present only a real impedance thatmatches the impedance of the source to the impedance of the antennaelement 174.

The goal with frequency matching is for a circuit to have a highefficiency and thus have little power dissipated within the matchingelements relative to the power that is transferred to the matcheddevice. This requires that the operating circuit have a much lower Qwhen loaded than the Q of the individual elements comprising thematching circuit. This is done by maximizing the unloaded Q of both ofthe capacitive elements in the circuit and the Q of any inductiveelements. As circuits are miniaturized, the reactance of the capacitiveelements becomes relatively high requiring high inductive reactanceswith correspondingly high Q. Inductors made with conventional materialshave lower Q's as they are reduced in size. However, nanotube Litz wireinductors have a unloaded Q that approaches the unloaded Q of thecapacitive elements, thereby making miniature higher frequency rangecircuits highly efficient and therefore feasible.

The embodiments disclosed above offer several advantages overconventional Litz wires, including: (1) the Litz wires have minimal skineffect, but are much stronger than metal wire-base Litz wires; (2) thethin cross-sectional area of nano-scale conductors allows for theproduction of extremely dense coils, which may be employed on integratedcircuits and in UHF applications; and (3) many nano-scale conductors,such as carbon nanotubes, move current by ballistic charge transport,which results in minimal heat generation through resistance and reducedline delay.

The above described embodiments, while including the preferredembodiment and the best mode of the invention known to the inventor atthe time of filing, are given as illustrative examples only. It will bereadily appreciated that many deviations may be made from the specificembodiments disclosed in this specification without departing from thespirit and scope of the invention. Accordingly, the scope of theinvention is to be determined by the claims below rather than beinglimited to the specifically described embodiments above.

1. An upper frequency-range circuit, comprising: a. a load elementexhibiting a capacitive load impedance; and b. a first matching networkthat includes at least one nano-scale Litz wire inductor and thatexhibits an inductive impedance that nominally matches the capacitiveload impedance.
 2. The high frequency circuit of claim 1, wherein thenano-scale Litz wire inductor comprises an inductor consistingessentially of carbon nanotubes.
 3. The high frequency circuit of claim1, wherein the capacitive load impedance comprises a miniature antennaelement.
 4. The high frequency circuit of claim 1, further comprising:a. an amplifier that exhibits a capacitive amplifier impedance; and b.an input nano-scale Litz wire matching network that matches thecapacitive amplifier impedance to a signal impedance.